CO₂ Capture By Membrane Process From Coal-fired Wet Desulfurized Flue Gas And The Failure Mechanism

Greenhouse effect caused by CO2 has become the global focus.Among numerous industrial CO2 emission sources,coal-fired power plant contributes the most amount of CO2.Membrane process is an effective method for CO2 capture from power plant.Generally,the membrane equipments for CO2 capture are suitable to installed downstream of wet flue gas desulfurization(WFGD)system.However,most of the existing studies are based on the simulated flue gas containing CO2/N2 only,and the researches that involve the performance and the operation of membrane for CO2 capture at actual WFGD conditions are relatively defective.The composition of desulfurized flue gas is complex containing a large of impurity such as fine particles,water vapor,SO2,except N2 and CO2.The fine particles and coexistent gaseous components in the flue gas may severely impact the performance of membrane or even to destroy membrane material.So far,there is little known specifically of the effect of impurity components and the influence mechanism.Therefore,this thesis focuses on effects of coexistent gaseous components and fine particles on membrane separation/adsorption properties and membrane materials under the condition of actual WFGD conditions.The systematic studies were conducted from the perspective of macro performance and micro mechanism.Effects of the coexistent gaseous components including SO2?SO3?water vapor?SOx/H2O and gypsum fine particles on the CO2 separation performance of polyimide(PI)hollow fiber membrane were investigated over membrane separation simulated test-bed and thermal WFGD simulated test-bed.Influence mechanism of gaseous and particles impurities on membrane properties and micro-structure were studied by using FESEM?EDS?AFM?FTIR.The results indicate that the individual present of low concentration of SO2 almost has no effect on the membrane separation for CO2;at high RH conditions(85?90%),the CO2/N2 selectivity increased slightly,while the permeation rate decreased gradually.The water vapor showed negative and reversible effect on PI membrane separation for CO2 on the whole.Compared with the case of the individual presence of water vapor,the coexistent of SO2/H2O and SO3/H2O,the CO2 separation performiance of PI membrane decreased more significantly.PI membrane became basically failure after operated 50 h under the coexistent of SO3/H2O.Fine particle could deposit on the surface of PI membrane and decreased the gas-membrane contact area then bring the irreversible destruction to the separation performance.During the process of CO2 capture from the actual coal-fired WFGD flue gas,the PI separation properties for CO2 capture still show a significant decrease,although the concentrations of coexistent gaseous components and fine particles were much lower than the case of simulated flue gas.The CO2/N2 selectivity and the permeation rate reduced by ca.90%and 45%due to the serious membrane fouling induced by the superimposed influence.The cake layer contained Al,Si,Mg,Ca,S,O,et al.Long-term membrane adsorption for CO2 capture was conducted both over membrane adsorption simulated test-bed and thermal WFGD simulated test-bed.Influences of the coexistent gaseous components and fly-ash fine particles on the membrane adsorption perfonnance for CO2 capture were investigated.Firstly,the sorption studies of SO2 on Polypropylene(PP),polytetrafluoroethylene(PTFE),polyvinylidene fluoride(PVDF)showed that PVDF was affected by SO2 more obvious than PP and PTFE.The effect of SOx?water vapor?SOx/H2O and NO/H2O on CO2 adsorption performance were comprehensively studied by using PP hollow fiber membrane.The results showed that the membrane adsorption for CO2 was not be impacted by the individual exists of SO2 and NO.However,the individual exists of water vapor reduced the properties of PP hollow fiber membrane for CO2 capture.And the influence became more significant with the increase of RH.Moreover,effect of water vapor was reversible.The coexistent of SO2/H2O and SO3/H2O would accelerate the performance deterioration of PP hollow fiber membrane.The influence of SO2/H2O on contact angle of membrane could be eliminated by blowing N2,while the destruction of mechanical strength and hydrophobic properties caused by SO3/H2O was permanent.The deposition of fine particles on the inlet of PP membrane module formed cake-layer,which became more significant with the presence of water vapor.The fine particles with particle size of D50=11 ?m and D50=27 ?m were introduced into the simulated flue gas to study the effect of particles sized.It was found that the adsorption performance decreased more with presence of particles with D50=11 ?m.And particle with smaller size deposited more distinctly.Furthermore,In addition,higher CO2 removal efficiency and less reduction were obtained under the tube side flow scheme.10 days experiment was conduct under actual WFGD conditions,PP membrane adsorption properties for CO2 capture still show a significant decrease.In addition,fine fly ash particles were observed not only adhering to the surface of the membrane but also entering into and blocking the membrane pores.On the basis of the influence of particulate-membrane fouling,to fully understand the mechanism of membrane fouling,a quantitative study of the adhesion force of particle on membrane surface was investigated by atomic force microscopy(AFM).The adhesion force of a single particle with flat glass,silicon wafer,PP(polypropylene)membrane,and inter-particles were measured.The influence of surface roughness,relative humidity(RH)and particle properties on the adhesion behavior were investigated.The results showed no obvious difference of adhesion force was obtained between the four substrates which has different surface roughness.And the surface roughness of flat substrate has slight effect on the adhesion force of the micrometer scale particle on flat surface at dry condition,while measured adhesion forces show obvious RH dependent for fly-ash and gypsum particles.Additionally,at dry conditions,the adhesion force of inter-particles also shows no obvious quantitative difference.The adhesion force of inter-particles increased more higher with the RH than that on membrane when RH higher than 65%,which indicates the adhesion between micrometer scale particles can accelerate the deposition of particles on membrane and contributes the most to membrane fouling in industry atmosphere.The adhesion force of gypsum particle was obtained larger than fly-ash particle.Van der Waals force,electrostatic force,capillary force between particle and membrane were calculated through classical force model.The results indicate:FL>Fvdw>Fe.In a dry environment,van der Waals force play a critical role in the particles adhesion force,while capillary force become dominated at wet conditions.Based on the water vapor-membrane fouling experiment,combined with novel molecular basket sorbent(MBS),the influences of silica support(SBA-15?TUD-1?HS-5)and PEG additive on the sorption performance of molecular basket sorbent(MBS)for CO2 capture.The results suggest that the pore properties of a support including 3D pore structure,pore size and the pore volume may not be directly related to the CO2 sorption performance of MBS sample.Instead,the pore properties of PEI-modified samples including 3D pore structure,pore size and the pore volume may play a more important role in the CO2,sorption capacity.MBS with 3D pore structure exhibits higher CO2 sorption capacity and amine efficiency than those with 2D-structured support.Among the sorbents studied,fumed silica(HS-5)based MBS showed the highest CO2 sorption capacity at the temperature range of 30 to 95 ?.It was found that the temperature dependence is directly related to the PEI surface coverage layers.The more PEI surface coverage layers,the higher diffusion barrier for CO2 and the stronger temperature dependence of CO2 capacity.Adding PEG could greatly promote the CO2 sorption capacity and improve amine efficiency of all MBS,most likely by effectively alleviating the diffusion barrier within PEI bulk layers through the inter-molecular interaction between PEI and PEG.The results may provide reference data for mixed matrix membrane preparation and modified.